1. Field of the Invention
The present invention relates to a screen printing method and apparatus.
2. Description of the Prior Art
In so-called screen printing, ink or a paste is squeezed out onto a substrate on which we want to print, which is placed on the opposite side of a screen plate obtained by forming a predetermined pattern on a metal or polymeric fiber mesh base with a photosensitive resin, through the screen plate by using a rubber, plastic, or metal blade called a squeegee, thereby transferring the pattern onto the substrate. Since screen printing can easily form a pattern on a relatively large area at a relatively low cost, the method has found more use over the years.
One characteristic way of using this method is its application to a large-area flat panel display typified by a plasma display. Application of the method to a plasma display demands high-precision printing quality as well as the capability of pattern formation on a large area in order to form a pattern of one million or more pixels on a large area of about 50 inches.
In the above screen printing method, as the squeegee moves on a screen plate while squeezing out ink or a paste onto a substrate to be printed (to be simply referred to as a work hereinafter) such as a printed board, the angle (to be referred to as the plate separation angle hereinafter) the screen plate (to he simply referred to as a plate hereinafter) separating from the work makes with this work upon completion of printing gradually decreases. This decrease in plate separation angle and the viscosity of ink or a paste make the plate difficult to separate from the work. That is, a degradation in plate separation properties occurs. This causes film thickness irregularity and line width irregularity, posing problems in screen printing for high-precision, large-area works.
For example, the above problems are traced back to irregularities which become more noticeable as the plate separation angle at an arbitrary squeegee position decreases in inverse proportion to the plate size with the distance (to be referred to as the clearance hereinafter) between the plate and the work remaining the same, and to low plate tension owing to a large plate size if the plate making technique level remains the same.
This problem may be effectively solved by ensuring a sufficient clearance. With an increase in clearance, however, the expansion of the overall plate increases. This increases the chance of irreversible expansion, resulting in short service life of the plate.
Efforts have been made to increase the plate tension by improving the plate making technique and the like. Even if, however, a high tension is attained, it is difficult to perfectly prevent a degradation in plate separation properties unless a certain plate separation angle or more can be maintained. As described previously, if the clearance remains the same, the plate separation angle at an arbitrary squeegee position decreases in inverse proportion to the plate size. It is therefore difficult to solve the above problem by only improving the plate making method, i.e., increasing the plate tension.
As a technique of improving plate separation properties without setting a large clearance, a printing technique (to be referred to as "off-contact printing" hereinafter) of improving plate separation properties by increasing the clearance of the plate in synchronism with the squeegee stroke, as shown in FIG. 1 in Japanese Unexamined Patent Publication No. 63-313895, has been proposed.
This technique will be briefly described below with reference to FIGS. 1A to 1D.
As shown in FIGS. 1A to 1D, a screen plate 1 stretched between screen frame members 1a and 1b and a work 2 are placed almost parallel to each other. In this case, the work 2 is fixed to a table (not shown) having a vacuum chucking mechanism and a high flatness precision. A pattern on the screen plate 1 is transferred onto the work 2 by squeezing out a paste placed on the screen plate 1 onto the work 2 with a squeegee 3 set in contact with the work 2 with a predetermined pressure.
One side of the screen plate 1 which is located on the start side of the squeegee 3 is raised in synchronism with the squeegee stroke, as shown in FIG. 1B. In this case, the screen 1 is linearly raised from the squeegee stroke start to the squeegee stroke end (see FIG. 1C).
The following is the reason why the screen plate 1 is linearly raised. With this operation, since two sides X and Y of the triangle defined by the screen mesh and the work surface after the plate separates from the work, which is indicated by the hatching in FIG. 1D, linearly change, geometrical similarity of the triangle is maintained. This allows printing, with a plate separation angle .theta. being kept perfectly constant, by setting the off-contact amount to an appropriate value in accordance with the clearance, thereby improving plate separation properties.
For example, FIG. 2 shows changes in plate separation angle .theta. (the ordinate) with respect to the squeegee stroke (the abscissa) when the off-contact amount is kept to 20 mm and the clearance is changed from 1 mm to 4 mm.
Note that the values shown in the graph of FIG. 2 are not measured values but are calculated values based on a simulation. As is obvious from this graph, when the clearance amount and the off-contact amount are set to 1 mm and 20 mm, respectively, the screen plate separation angle .theta. is constant.
As a contact printing technique, a technique using a mechanism like the one described in Japanese Unexamined Patent Publication No. 7-309003 has been proposed, with the same contents as those described above.
For the same purpose as that described above, a technique of improving plate separation properties by slowly lowering the work after or in synchronism with the squeegee stroke, as disclosed in Japanese Unexamined Patent Publication No. 5-185580, has been proposed. This technique has almost the same effect as that of the above method from the viewpoint of keeping the plate separation angle .theta. constant by slightly increasing the clearance.
In addition, in printing on a large area such as a plasma display, the printing area tends to become large with respect to the outer size of the plate frame as compared with conventional screen printing for the following reason. If a screen plate is to have a size with a margin on the peripheral portion as in the prior art, a large screen plate having a side with a length of about 2.5 m and a large printing machine on which this screen place can be set are required. This makes it difficult to manufacture plates and printing apparatuses with high precision.
A large printing area with respect to the outer size of the plate frame makes it difficult to print by only using a portion near the central portion of the screen plate which is high in tension uniformity and having a sufficient expansion margin. Since the pressure which the squeegee exerts on the work, i.e., the printing pressure, changes between the peripheral portion of the plate which has a relatively high tension and the central portion which has a relatively lower tension, film thickness irregularity and line width irregularity occur. In order to solve this problem, the clearance is minimized, a sturdy yet flexible squeegee is used, or the squeegee is placed upright in many cases.
Assume that since the expansion margin of a screen plate is not sufficient, it is used to print while the clearance is increased to some extent to ensure good printability. In this case, the expansion of the screen plate becomes excessive, resulting in a decrease in printing precision. This often leads to irreversible deformation of the screen plate and short service life of the screen plate. In many cases, such a problem can be prevented by setting a minimum clearance as a printing condition.
According to the conventional method described above, the plate separation properties of the screen plate can be greatly improved, but irreversible expansion of the plate cannot be completely prevented because the clearance increases as the plate is separated from the work although the clearance is small at the start of printing. In addition, a new problem develops. That is, since the clearance at the squeegee stroke start greatly differs from that at the squeegee stroke end, the pattern formed deforms unevenly in printing operation.
FIG. 3 shows changes in the expansion amount of the screen plate when the off-contact printing mechanism shown in FIGS. 1A to 1D is used. The values shown in FIG. 3 are not measured values but are calculated values.
The conditions given in this calculation are:
Off-contact operation: Off-contact operation is performed in synchronism with the squeegee stroke. When the squeegee moves by a stroke of 1,700 mm, the plate end portion on the squeegee start side is raised to a level of 20 mm.
Screen plate: The screen plate has an inner dimension of 1,340 mm. The screen mesh portion between screen frame members and the squeegee expands in the form of a triangle.
The expansion amount of the screen plate in the squeegee stroke direction is calculated when the squeegee is moved while the clearance value is changed to 0, 1, 2, 3, and 4 mm. Referring to FIG. 3, the squeegee stroke is plotted along the abscissa, and the screen plate expansion amount is plotted along the ordinate.
As is obvious from FIG. 3, the expansion amount change of the screen plate is relatively small in a region where the clearance is small (0 to 1 mm). However, as the clearance increases (2 mm or more), the expansion of the screen plate abruptly increases. As a result, the difference between the expansion amount on the squeegee start side and that on the squeegee end side is especially noticeable.
When a conductive paste (silver paste NP-4028 available from NORITAKE CO., LTD.) was printed on a soda-lime glass substrate in a 40-inch electrode pattern under the above conditions by using a 1,500-mm square screen plate (inner dimension: 1,340 mm) with so-called combination lining (available from Tokyo Process Service K.K.), obtained by covering the peripheral portion of a plate with a polyester printing mesh and also covering the inner portion of the plate with a stainless printing mesh (SX300 mesh was used in this case), good plate separation properties were observed in a case wherein the clearance was 4 mm or more (the squeegee stroke speed was 20 mm/s).
Of the conditions shown in FIG. 3, the condition indicated by the curve with a clearance of 4 mm allows printing while satisfactory plate separation properties can be ensured. With a smaller clearance amount, film thickness irregularity and a deterioration in surface state are caused by a deterioration in plate separation properties. That is, high-precision printing under this condition is not practical.
Although the effect shown in FIG. 3 can be obtained when the practical clearance is set to the minimum value, i.e., 4, since uneven pattern deformation occurs in off-contact printing shown in FIGS. 1A to 1D, it is difficult to use this technique for the manufacture of a large plasma display.
In order to prevent uneven expansion, the plate separation properties on the squeegee end side can be improved by moving the plate or the work in the direction in which the clearance increases upon completion of a squeegee stroke. With this technique, however, since irregularity due to a deterioration in plate separation properties during the squeegee stroke cannot be eliminated, a radical improvement cannot be expected.
Although printing with zero clearance, i.e., so-called contact printing, produces no irregularity due to a deterioration in plate separation properties, the paste spreads, and uniform separation of the plate from the work is difficult. This technique cannot easily provide a solution either. For this reason, in the off-contact screen printing method shown in FIGS. 1A to 1D, a serious problem is experienced in micromachining on a large area such as a plasma display.
According to a method of preventing printing irregularity on the peripheral portion of a screen plate with a large pattern size by changing the printing conditions, since main conditions for printing, such as clearance, squeegee hardness, and squeegee angle are limitated, the working margin reduces, resulting in low yield.
In addition, in the method of preventing printing irregularity on the peripheral portion of a screen plate with a large pattern size by changing the printing conditions, the clearance must be suppressed. This limitation reduces the working margin, resulting in low yield again. This method can be improved by using the above off-contact printing technique together because the initial clearance can be minimized. However, since the clearance actually increases on the squeegee end side, irregularity and uneven expansion of the screen plate due to variations in printing pressure become worse.